Since a Si waveguide is strongly resistant to bending, it is prospectively used as an optical waveguide in a microscopic region. In addition, a light-receiving element that can be integrated with the Si waveguide is being sought. Characteristics required in such a light-receiving element include reduction of a dark current, highly efficient photoelectric conversion, and easy integration with the Si waveguide.
Reduction of the dark current is important to improve reception sensitivity. In a light-receiving element having a large dark current, an electric current flows even when light is not incident. This becomes a noise component and generates degradation of the reception sensitivity due to the determination of the S/N ratio. Generally, the light-absorption layer of the light-receiving element combined with the Si waveguide is made of Ge. However, since the Ge has a different lattice constant from that of Si, when the light-absorption layer is integratedly formed on the Si, the dark current increases due to lattice defects or the like.
In addition, highly efficient photoelectric conversion is also important. Particularly, the reception sensitivity is associated with efficiency of the element. As the efficiency increases, it is possible to obtain a higher reception sensitivity. As an example of methods for improving the efficiency, it was proposed to employ an Avalansche photo diode (APD) structure having an amplification function. A high voltage is to be applied to the element to obtain an APD operation. However, in this case, since a high electric field is applied to a Ge absorption layer having a lot of defects, the dark current increases, and the sensitivity is degraded.
As described above, development of a highly efficient element having a low dark current and a high integration capability with the Si waveguide as an optical guide is sought.
In Non-patent Document 1, a Ge photodetector integrated with the Si waveguide using a selective growth has been reported. FIG. 11 is a perspective view illustrating a light-receiving element described in this report. In this light-receiving element, an intrinsic Ge layer 201 as an absorption layer is formed on the rib type Si waveguide 203 using a selective growth, and an n-type Ge layer 202 as a contact layer is formed thereon. An electrode 206 is formed on the n-type Ge layer 202, and an electrode 205 is also formed on the Si waveguide 203. The Si waveguide 203 is a p-type. As described above, when Ge is grown on Si, because both elements have different lattice constants, crystallinity of Ge is not satisfactory. In this document, since the intrinsic Ge layer 201 is selectively grown only in the microscopic region on the Si waveguide 203, generation of lattice defects within the intrinsic Ge layer 201 is suppressed.
[Non-patent Document 1] 68th Japan Society of Applied Physics Meetings, Pamphlet No. 0 P. 89 6p-C-7, “Ge photodetectors integrated with Si waveguides via selective epitaxial growth,” by Sung-bong, Park